The invention relates to well treatment processes for acidizing subterranean reservoir formations. It is particularly useful for treating reservoirs which are relatively hot and contain significant proportions of acid soluble calcium compounds.
It is known that aqueous solutions of strong acid dissolve significant proportions of ferric and ferrous iron from iron containers or conduits or components of subterranean reservoir formations. For example, a paper of Journal of Petroleum Technology, September 1969, pages 1121 to 1129, indicates that the dissolved iron content in spent acids from wells treated in West Texas was significantly high and, in some cases, about 2.6 percent. It states that "Sequestering agents have been used for many years to prevent the reprecipitation of iron from spent acid solutions. Those most commonly used include citric acid, acetic acid, mixtures of citric and acetic acids, lactic acid, gluconic acid, ethylenediaminetetraacetic acid (EDTA), and nitrilotriacetic acid (NTA)." And, "The EDTA, NTA, and citrate complexes of iron are much more stable than the acetate."
Patent application Ser. No. 748,087, filed Dec. 6, 1976, now U.S. Pat. No. 4,096,914, by W. A. McLaughlin and E. A. Richardson (the 087 application) indicates that, in addition to being capable of forming permeability impairing hydrated iron oxides, ferric ions in an oil reservoir acidizing fluid are also capable of forming permeability impairing iron-asphaltene solids, particularly in reservoirs containing more than about 1/2% by weight of asphaltenic components. The solids formed by the interaction between ferric ions and asphaltenes are particularly insoluble. Although none of the prior art located before the 087 application was filed mentioned problems due to solids formed by the interaction of iron and asphaltenes, the difficulties of removing organic deposits containing asphaltenic materials were known. For example, the need for using solvents containing both an aromatic hydrocarbon and an amine in order to dissolve asphaltene-containing deposits was described in application B-359,946, published Jan. 28, 1975 (now U.S. Pat. No. 3,914,132).
The 087 application indicates that the forming and precipitating of iron-asphaltene solids due to the interaction of a ferric ion-containing aqueous acid and a crude oil is not readily apparent, due to the relatively slow precipitate development and emulsion-forming tendencies of such a system. However, based on many tests of numerous types of reducing agents and iron chelating materials, that application was premised on a discovery that the incorporation of salicyclic acid within a ferric ion-containing aqueous solution of a strong acid was uniquely effective in preventing the formation of permeability impairing iron-asphaltene solids.
Numerous patents relate to using chelating agents for avoiding the deleterious effects of dissolved iron in well treatments involving aqueous strong acids. For example, in U.S. Pat. No. 2,175,081, which was applied for in 1937, it is suggested that most of such deleterious effects are due to the precipitation of ferric ions, and such effects can be avoided by incorporating a water-soluble reducing agent within the injected acid. U.S. Pat. No. 2,175,095 suggests the inclusion within an acidizing fluid of a material such as lactic acid, ammonium acetate, glycine, glycolic acid, citric acid, or the like, which is capable of preventing the precipitation of iron or aluminum hydrates at their normal precipitation pH values. U.S. Pat. No. 2,335,689 suggests a similar inclusion of an iron sequestering agent, such as a polyhydric phenol within the injected acids. U.S. Pat. No. 3,142,335 suggests such a use of a sequestering agent containing a mixture of ingredients that function as a buffering agent, such as citric acid or citrate salt mixed with acetic or formic acids or their salts. U.S. Pat. No. 3,150,081 suggests s similar incorporation of mixtures of hydroxyacetic and citric acids, which mixtures are said to be cheaper and more effective than such a use of either of those acids by itself.
Patent application Ser. No. 809,586 filed June 24, 1977, now U.S. Pat. No. 4,137,972 by W. A. McLaughlin and D. C. Berkshire describes a discovery that with respect to chelating ferric ions that become dissolved in a strong acid solution in order to keep those ions from forming iron-asphaltenic solids, 5-sulfosalicylic acid was found to be more beneficial than salicylic acid, for the following reasons: the 5-sulfosalicylic acid (a) chelates with more ferric ions at a lower mole ratio of chelating agent to ferric ions at pH's of less than about 3, (b) has a much greater water-solubility at the lower pH's and (c) forms calcium salts which are significantly water-soluble and are capable of avoiding any deleterious precipitation of calcium sulfosalicylate.